Nowadays the paper made of natural pulp is replaced with the polyolefin synthetic paper mainly comprising the basic substrate layer (intermediate Layer) in the intermediate Layer called biaxially oriented polypropylene (hereinafter, abbreviated as BOPP), whereon the back surface is laminated with the uniaxially oriented polypropylene (UOPP) containing inorganic fine powder as the paper surface layer (such as the Japanese Patent publication No. 40794/71, Japanese Laid-open Publication No. 141339/81). Moreover, for improving gravure printability, coating composition comprising liquid acrylic copolymer and polyethyleneimine can be coated onto the surface to give 0.005-0.1 gIm.sup.2 (the Japanese Laid-open Publication No. 10624/75, No. 161478/75); The Japanese Laid-open Publication No. 87255/91 discloses a composite film of opacity over 90% made from a biaxially oriented polymer film to serve as a substrate layer (intermediate layer) which is formed by resin composition of high crystallinity polypropylene in isotacticity over 98% of 35-95% weight and talc and/or mica powder of 5-65% weight, and a uniaxially oriented polymer film adhered hereonto at least one surface of the intermediate layer by resin composition of high crystallinity polypropylene in isotacticity over 98% of 20-90% weight and inorganic filler powder in 10-80% weight.
Such a synthetic paper can contact closely with printing head, and provides the merits of rigidity-mechanical strength, but leaves the drawbacks on the ink drying speed. The amount and crosslinking of synthetic paper pore is inferior to that of cultural paper. As a result, printing ink takes quite a long time before completely dried, that is why it is unsuitable for cultural paper (in the case of 4-colour printing it takes 8 hours for drying). Also, the amount of stacking is limited to prevent unneeded reverse printing. Thus, it will increase the man-hours and space for drying in the 4-color printing process. Furthermore, due to the surface layer is the uniaxially oriented polypropylene firm containing inorganic fine filler, poor transfer of ink upon offset printing often occurs in multicolor continuous offset printing and also in both surface printing.
This poor ink adhesiveness must be improved by increasing the ink outlet to reach the required color saturation. Meanwhile, to avoid the deformation of paper surface and the variation of paper dimension by the ink solvent, the specific ink is used for printing which is 3-4 times price of those used in the natural paper.
These drawbacks mentioned above specify the different expense during the printing process between the uniaxial oriented polypropylene synthetic paper and the natural paper also explain why the natural paper can not be replaced in various uses.
Nissin textile Company (Japan) produces the synthetic paper by forming a layer with surface treatment in the extensible or non-extensible films of PP, polyvinyl chloride, Polystyrene, and Polyester. The properties on dryness and printability of the afore paper though are far better than those of the uniaxial oriented PP synthetic paper containing inorganic filler. In general conception, it should be able to replace the natural paper extensively. As a matter of fact, this synthetic paper costs higher than those of various items of natural paper Accordingly, its market sale is inferior to that of the uniaxial oriented PP synthetic pa-containing inorganic filler. In its manufacturing process, it utilizes DMF as the solvent for the surface treatment, and causes a large amount of poison water, which subsequently demands a costly investment for dealing with poisonous drainage to prevent great detriment to environment. In addition, the reverse coater required for using the DMF solvent system will cause a quite thick paper layer (about 30.mu. for general type and 10-20.mu. for special product use), not available for coating layer under 10.mu. with excellent paper surface quality.
The aforesaid factors will certainly increase the cost (such as materials, facilities) during the coating process, as a result, not suitable for extensively adopting synthetic paper to replace of natural paper except for some specific use.
In order to produce a low-cost synthetic paper without drawbacks mention above to be able to extensively replace of natural paper in cultural paper use, the applicant had spent quite a long time for thinking, and finally got a Taiwanese patent, No. 85100270, which mainly use a method such as co-extrusion in which the different melts of polymers are extruded from one primary extruder and two secondary extruders separately prior to extruding them simultaneous through a single main "T" die for forming a three-layer sheet and then through cooling and shaping, biaxially orientation, electric halo treatment and laminating/rolling to be a synthetic paper with required thickness. At last, surface treatment for coating on the paper surface obtains a biaxially oriented PP synthetic paper of easy drying printability.
The synthetic paper made from the method of Taiwanese patent No. 85100270 is almost the same as art paper in using of kind and amount of inks, the dryness and quality of printings, and the amount and crosslinking of minute pore. Its some features are superior to traditional paper, but the surface gloss and the color vividness are inferior to them. The original surface of art paper is rugged which is required for pressed several times for smoothing.
In order to solve the drawbacks for being unable to raise up the gloss of paper, quality of surface and its printings, and the problem for the big loss of paper thickness during the foggy facing process of art paper which prevents the endeavor for the improvement of paper ruggedness (now, the best ruggedness of paper in the world is 0.8-0.9.mu.), the applicant of present invention experienced a number of experiments trying to figure out a different way not only from the existing arts but better than them. The applicant finally creates the invention which provides merits over traditional synthetic papers in product cost, quality (such as surface gloss, printing gloss, ink drying speed), and over art paper in specific gravity, surface ruggedness, and printing resolution.